Sigma-delta modulators are used for analog to digital conversion. Sigma-delta modulators offer high resolution, high integration and low cost, making them an ideal choice for many applications where analog to digital conversion is required.
The operation of a sigma-delta modulator is best described using the simplest 1-bit implementation. A typical 1-bit sigma-delta modulator is illustrated in FIG. 1.
In the basic implementation illustrated in FIG. 1, the sigma-delta modulator comprises a summation unit 1, an integrator 2, a comparator 3 and a digital-to-analog converter (DAC) 4. The summation unit may be, for example, a difference amplifier. The comparator may be an analog-to-digital converter (ADC).
As can be seen in FIG. 1, the components of the modulator are connected in a feedback loop. The analog input signal is fed into the summation unit, where a feedback signal is subtracted, before being fed into a loop filter, which in this case is an integrator. The signal output from the integrator is compared with a reference signal in the comparator. If the output signal from the integrator is greater than the reference signal, a ‘one’ is output, and if the integrator output signal is less than the reference signal, a ‘zero’ is output. Thus the analog input signal has been converted into a digital output signal.
The digital output signal is fed back, via the DAC, to the summation unit, where it is subtracted from the input signal. The purpose of the feedback signal is to maintain the average output of the integrator near the comparator's reference level by making the ones and zeros of the digital output signal representative of the analog input.
The DAC in the feedback loop has an upper reference voltage and a lower reference voltage. When the comparator outputs a ‘one’, the DAC outputs a signal at the upper voltage and when the comparator outputs a ‘zero’, the DAC outputs a signal at the lower voltage. The modulator is at full-range scale when the input signal is equal to the upper or lower reference voltages of the feedback DAC. For example, if the feedback DAC outputs −2.5V when it receives a zero and +2.5V when it receives a one then the range of the input is ±2.5V. The reference voltage of the comparator is halfway between the upper and lower boundaries of the input range, e.g. for an input range of ±2.5V the reference voltage for the comparator would be 0V. The reference voltage for the comparator represents the virtual ground level for the modulator. For modulators having an input range that is symmetrical about zero, the virtual ground level is zero.
The output from the sigma-delta modulator is a stream of ones and zeros. The ratio of ones to zeros represents the magnitude of the input signal compared with the input range of the modulator. For example, if the range of the modulator is ±2.5V and the input signal has a magnitude of 1.0V, then the input signal is 3.5V above the lower boundary of a 5V range. In this example, 70% of the output signal should consist of ones. For the modulator to produce a digital output signal that is an accurate representation of the analog input signal, the modulator must sample at a much greater rate than the rate of change of the analog input signal.
More sophisticated sigma-delta modulators than the 1-bit modulator described above may have multiple modulators and integrators.
A sigma-delta modulator offers improved noise performance over traditional ADCs. This is achieved through oversampling, noise shaping, digital filtering and decimation.